Field
The disclosed subject matter relates to the field of chemical recycling of mixed waste plastics. More particularly but not exclusively, the subject matter relates to thermal or thermo-catalytic degradation of mixed waste plastic feedstock to obtain lower molecular weight hydrocarbon products.
Discussion of Related Field
Several conventional technologies have attempted thermal or thermo-catalytic pyrolysis of heterogenous waste plastic feedstock to obtain lower molecular weight hydrocarbon products. Generally, mixed waste plastic feedstock obtained from municipal, commercial, industrial and agricultural waste streams comprises polymer resins, such as, high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyethylene terephthalate (PET), as well as, other engineering resins, and may be contaminated with some amount of metal, cellulosic biomass, paper, organic matter, dirt and other materials. The mixed waste plastic feedstock is suitably pre-processed to obtain a hydrocarbon rich stream, which is then pyrolysed. The pyrolysis of the hydrocarbon rich mixed waste plastic stream results in formation of a wide range of hydrocarbon products. The resultant hydrocarbon products can be divided into three major streams, namely, a volatile gas fraction, a liquid petroleum fraction and a solid residue fraction. The volatile gas fraction comprises hydrocarbon chains, each having 1 to 5 carbon atoms. The liquid petroleum fraction, which includes paraffins, olefins, iso-paraffins, naphthenes and aromatics, comprises hydrocarbon chains, each having at least 5 carbon atoms. The solid residue fraction comprises carbon char and other non-volatile reaction products coming from the pyrolytic process.
To enhance the commercial profitability of chemical recycling of mixed waste plastic feedstock, it is desirable that a substantial part of the generated liquid petroleum fraction be physicochemically similar to distillate fuels, such as, gasoline, kerosene and diesel. It is well known that such distillate fuels comprise hydrocarbon chains, which mostly have 5 to 24 carbon atoms linked together. However, it has been observed that, chemical recycling of mixed waste plastic feedstock often yields a very broad compositional range of liquid hydrocarbon mixtures, starting from C5 (n-, iso and neo-pentane) and extending all the way up to C80 and beyond. Such resultant product is often referred to as ‘synthetic crude’ and it includes a substantially large amount of petroleum waxes and slurries, which comprises hydrocarbon chains having over 24 carbon atoms linked together. Consequently, such synthetic crude oil has high viscosity, low API and a lower economic value compared to distillate fuels, such as, gasoline, kerosene, and diesel.
For example, U.S. Pat. No. 7,758,729 describes a method for degradation of mixed waste plastic feedstock wherein “plastic material is heated in a vertical kiln treatment chamber in incremental steps through a series of graduated temperature set points wherein each graduated temperature set point corresponds to a vaporization temperature of an individual by-product of said plastic material, and pulling a vacuum of inert gas on the treatment chamber at each temperature set point to selectively remove an individual by-product corresponding to the temperature set point.” Application of such a method converts mixed waste plastic feedstock into high viscosity ‘synthetic crude’ oil having a carbon number distribution which extends from C5 to C80.
In light of the foregoing discussion, there is a need for an improved technique to chemically recycle mixed waste plastic feedstock, for generation of lower molecular weight hydrocarbon products.